This invention relates to electrokinetic flow and, more particularly, to the use of electrokinetic flow to transport particles.
When a liquid flows slowly through a cylindrical tube, propelled by gravity or by an applied pressure difference between the ends of the tube, a parabolic laminar (Poiseuille) flow velocity profile is established transverse to the direction of flow. The flow velocity is a maximum along the central axis of the tube and approaches zero at the tube wall. When such a moving liquid is used to transport particles from one location to another, the particles will have a range of transit times that depends upon their radial distance from the tube axis. Particles near the tube axis will travel faster and may overtake particles traveling near the wall, thereby scrambling the order of the transported particles. Moreover, small particles or molecules released into the stream will diffuse radially across zones of different velocity in an unpredictable fashion. Thus, even if such particles start their travel at the same point in the tube and at the same time, they will arrive at the end of the tube at different and unpredictable times.
The inability of a flow stream with a Poiseuille flow velocity profile to deliver small particles or molecules (all referred to herein as particles) in a predictable manner presents a serious problem for applications that require particles to arrive at the far end of a transport tube after a predictable transit time or in a predictable order. For example, a method for sequencing DNA, described in U.S. Pat. No. 4,962,037, requires that individual DNA bases be transported via a flow stream from a zone in which enzymatic cleavage is occurring into a DNA base detector. The individual bases must arrive at the detector in a known sequence so that the DNA sequence can be reconstructed. Sequence errors can be avoided by reducing the number of bases in the transport flow stream, but only at the cost of a major reduction in sequencing rate.
In another aspect of particle sequencing and identification, particles are individually tagged with fluorescent labels (dye molecules that emit a characteristic fluorescence when excited with a corresponding laser). Different dyes are required to identify individual ones of the particles with a concomitant capability to excite and detect fluorescence from the various dyes. It would be desirable to provide only a single dye for use in identifying particular particle species.
These problems are addressed by the present invention wherein electrokinetic flow is used for particle transport. Electrokinetic flow includes an electroosmotic flow component for maintaining a continuous ordered flow of particles and an electrophoretic flow component for providing a velocity differential that uniquely identifies the particles by species.
Accordingly, it is an object of the present invention to provide a flow stream capable of maintaining an ordered stream of small particles and/or molecules.
It is another object of the present invention to maintain a plug flow profile across a capillary tube and along the length of the capillary tube.
Yet another object of the present invention is to provide a predictable electroosmotic flow rate to calculate an electrophoretic velocity from a measured total particle flow rate for use in particle identification.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.